Ok, here is a thinking out loud moment...
During puberty I and many of my peers felt out of place, not understood, wanted to change the world, and wanted to rebel against expectation.
It was what teens did, but as every prior generation before us had done, we felt we were the real deal and really meant it this time...
I fast forward to today, and there is a new way to protest, to rebel and to escape (seemingly). A way to escape the mixed up feelings you have. It's to escape your sex and become a different gender. To a teenager it is a plausible explanation for the feelings inside. It's an attractive proposition to dabble in being the opposite gender, and it's a cause worth fighting for...
It is so compelling, many teens have chosen this option, just as former generations chose free love, or green hair. Only this time, the course of action can result in permanent life changes.
In some ways my thoughts might sound like I am reducing transexuality to passing fad. A result of teenage angst. And whilst for some it could be a far more complex psychological or physiological phenomenon, could it not be true that transexuality is the current generation's cause?
If a teen is not happy with their life, the new option is to radically redefine it, not to wait for it to evolve.